Polycarbonate resins have high impact resistance with ductility to notch or crack propagation at an average of up to about 0.2 inches thickness when the incident notch is 10 mils (thousandths of an inch) in radius. Above this average thickness the impact resistance and ductility of polycarbonate resins decline. This phenomena is commonly found in glassy plastics and is referred to as the critical thickness for notched impact resistance of a glassy plastic.
In addition, the impact strength of notched polycarbonate resins decreases as temperatures decrease below about -5.degree. C. and also after aging the polymers at elevated temperatures above about 100.degree. C. These temperatures are commonly found in applications where extremes of heat and cold are to be expected.
Thus, it is desirable to use a composition which extends the impact strength and ductility of polycarbonate resins to variable thickness parts or articles of use which resist embrittlement upon exposure to high or low temperature in a notched or scratched condition.
Compositions are known which extend the high impact resistance and ductility characteristics of polycarbonate resins to parts beyond the critical thickness and under low and high temperature aging conditions, but many of these compositions suffer from incompatibilities of the polymeric components which results in poor weldline or knit line strength in fabricated parts as evidenced by low double-gate impact strengths when measured according to ASTM D256.
It has now been found that the addition of a linear low density polyethylene, and an alkenyl aromatic copolymer to a polycarbonate resin results in a molding composition that may be utilized to make molded articles having improved weld line strengths when measured in accordance with ASTM D256, and improved heat stability as compared to prior art compositions that were based on a combination of a polycarbonate and a high density polyethylene or a low density high pressure polyethylene. In addition, these molding compositions generally have reduced splay or plate out, a wider temperature molding profile, less shear sensitivity, improved hydrolytic stability and enhanced chemical resistance as compared to compositions that contain high density polyethylene or low density high pressure polyethylene.
The preferred compositions according to the present invention will have a weld line strength according to ASTM D256 of greater than about 7.0 ft/lbs. and preferably above about 12.0 ft/lbs.